The OSI-RE-SS is a reflector- type linear optical beam smoke detector designed to work being a part of fireplace alarm devices. The beam operates totally on the theory of light obscuration utilising an Infra-Red beam. OSI-R-SS is usually a four-wire conventional reflector-kind linear optical beam smoke detector to be used https://www.firesense.co.uk/type/conventional-fire-system/conventional-beam-detectors/